In most all-purpose two-cycle engines used as sources of motive power in small vehicles and portable machinery for agriculture and forestry, etc., fuel is supplied by means of a diaphragm type carburetor equipped with a constant-fuel chamber. The constant-fuel chamber is generally separated from the atmosphere by a diaphragm which adjusts the fuel to a constant pressure.
A diaphragm type carburetor is also equipped with a diaphragm type fuel pump driven by pulse pressure generated in the crankcase of the engine. Fuel from the fuel tank is introduced into the constant-fuel chamber by this fuel pump, and is drawn into the air intake passage and supplied to the engine from this constant-fuel chamber. Such a carburetor is ordinarily equipped with a manual starting pump which feeds an extra amount of fuel out into the air intake passage or introduces a specified amount of fuel into the constant-fuel chamber prior to the starting of the engine. The operation of the manual starting pump improves the starting characteristics of the engine at low temperatures.
In diaphragm type carburetors, it has long been known to regulate the fuel flow rate by the installation of a main jet in the fuel passage leading from the constant-fuel chamber to the main nozzle, or by the promotion of fuel atomization and limiting of the fuel flow rate by the introduction of bleed air into the fuel in the fuel passage. An example of those techniques is disclosed in Japanese Patent Application Kokoku No. Sho 46-10565.
Prevention of the flow of air into the constant-fuel chamber from the main nozzle during engine deceleration by the installation of a check valve in the main nozzle or fuel passage has long been known in diaphragm type carburetors. An example of this technique can be found in U.S. Pat. No. 3,404,872 or Japanese Patent Application Kokai No. Sho 55-69748.
All-purpose two-cycle engines equipped with diaphragm type carburetors generally have a single cylinder, so that the air flowing through the air intake passage undergoes a pulse motion. It is known from experience that the supply of fuel from a main nozzle opening into the narrowest part of the venturi of the air intake passage is generally greater in the case of a single-cylinder engine in which the air flow is intermittent than in the case of a multi-cylinder engine in which the air flow is continuous. Accordingly, diaphragm type carburetors for use in all-purpose two-cycle engines are constructed so that the air flow velocity in the narrowest part of the venturi is lower than in a carburetor meant for use in multi-cylinder engines.
The main nozzles of diaphragm type carburetors proposed in the past have a nozzle opening at the tip end of the main nozzle. The nozzle configurations of current designs have a nozzle opening at a point which is in the same plane as or protruding slightly from the wall surface of the narrowest part of the venturi. As a result, the fuel which is sucked out by a low-velocity air flow, and thus by a low venturi negative pressure, tends to flow along the wall surfaces, so that sufficient atomization is difficult to achieve even if bleed air is introduced. This leads to uneven engine revolution and insufficient engine output. Especially in carburetors in which a check valve is installed in the main nozzle, poor atomization is achieved because this check valve hinders the atomization of the fuel, resulting in poor combustion when the fuel is first sucked out.
The present invention is intended to provide a diaphragm type carburetor which solves the aforementioned problem of difficult fuel atomization encountered in conventional diaphragm type carburetors, so that an appropriate amount of fuel can be sufficiently atomized and supplied to the engine at all times.